Tide staff: The gold standard in tide gauging
Tide staff readings of sea level provided the first tide gauge data. Among the oldest known are those of Brest. From 1711 to 1716, these measurements, taken by day in the vicinity of high and low tides, were
Using the tide staff
The tide staff can be used as an aid to navigation. Currently, its primary purpose is to calibrate and control the water depth measured by other tide measurement systems.
To comply with the hydrography standards for measuring sea level (standards defined by the International Hydrographic Bureau) a permanent station must include a tide staff. It is the only instrument that provides a direct reading of the level outside the well, which makes it the key instrument for verifying the level measured inside the well (proper functioning of the well and validation of the measurements recorded). To ensure proper verification, the tide staff should be located in the immediate vicinity of the tide station.
Common sense must be used when installing the tide staff. Tide staffs must be built from materials that resists corrosion and can be easily cleaned to ensure correct reading of the scale. They should be placed away from risk areas (destruction or deterioration) or areas hidden by ships at berth. Tide staffs should also be vertical. If this is impossible, (in the case of non-vertical quays or piers), the scale must be adjusted to determine the actual heights.
Although it is not an absolute rule, the the staff zero should be adjusted to the chart datum to avoid misinterpretation by uninformed users.
In areas with high tidal ranges, it may be impossible for one tide staff to cover the entire height variation. In this case, at least two tide staffs are required; the high tide staff should be placed at a dock, and the low tide staff at a location that remains submerged.
Example of a tide staff
Here is an example of a tide staff:
The scale is a series of red or black squares placed every 10 cm, separated by a white interval of the same size. The squares are grouped by alternating series of three squares of the same colour, for easy estimation of one meter. Resolution of 10 cm may seem crude, but it is easy to evaluate the quarter scale (± 2.5 cm). Better precision would be difficult to achieve, given the waves and choppiness. The height measurement indicated by the tide gauges are adjusted by averaging multiple readings of the tide staff. Other types of scales are used by other organisations.
Reading the tide staff
Reading the tide staff is sometimes difficult, especially in the presence of waves. Common practice is to take the average of successive extreme (high and low) values. However, because the waves do not have a sine wave form but more of a trochoidal shape (pointed crests and rounded troughs), this method gives a higher value than the actual level.
Taking into account the angle of a quay
Quays are not always perfectly perpendicular to the water. There may be an angle between the quay (red line) and the vertical plane (blue line). The angle between the steepest slope of the quay and the vertical line introduces an error in water level measurements when using:
- a water level indicator tape that runs down the quay;
- a tide staff without taking into account the angle of the quay.
It is necessary to correct for this angle to obtain the actual air draft.
When does this angle have to measured?
The angle should be measured in the following cases:
- for a new tide staff:
- for a tide staff that has not been used for a long time;
- if there is a doubt as to the accuracy of the previous angle measurement.
The angle used is angle α between the plane of the quay and the vertical line.
How is this angle measured?
The principle is simple: place a long ruler (at least 2 meters) against the wall of the quay, with part of the ruler extending beyond the quay.
Take two measurements:
- the length of the part of the ruler which extends beyond the quay (D);
- the vertical distance from the end of the ruler to the quay (h) using a plumb line.
The angle of the quay is the arccosine of h over D: α = arcos(h/D)
Tidal bench marks
The tide staff zero is referenced to at least three fixed bench marks on land. They are located far enough from each other to avoid simultaneous destruction, for example during port construction work. One is selected as the primary bench mark; a levelling bench mark is usually recommended. Their respective values, relative to the chart datum and levelling (if any) are recorded in a record with drawings and photos so they can be easily located.
To find out more:
- Simon B. (2007). La Marée - La marée océanique et côtière. Edition Institut océanographique, 434pp.
Last updated: 12/12/2012